Fence Posts - Sonotubes?

Hi All,

I am preparing to install a wood fence in Ontario, Canada. I'm using 6x6 PT posts. My main goal is doing this right - we're dropping a good chunk of change on this project and I want something that will last a good long time.

Current plan is:

1. Dig down 4'.
2. Add 4-5 inches of gravel, and tamp down.
3. Add sonotube and place post in hole. Leave 2 inches of sonotube above ground.
4. Fill sonotube with concrete, and bevel the top so water drains away from the post.

Is this the right way to go about it? I've heard conflicting bits on whether to use sonotubes or not. I think, given how cold it gets during the winter that the nice smooth surface the sonotube gives will provide less area for frost to "bite" into and heave.

Also - do I leave the sonotube in place once the concrete is in, or pull it out after some amount of time?

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Fence Posts - Sonotubes?

One advantage of the sonotube is that you use a minimum amount of concrete, especially in certain types of soil where over-excavation is common. If you have a strung-out job, you certainly do not want to haul out too much material and want to backfill with what is there.

In areas where there is frost, the smooth forms definetly resist the uplift from the surrounding surface frost since there is some "slip" that does not occur with the rough dug surface.

Keep the top of the concrete above ground level.

You can leave the forms on - you will never be able to pull them out. You can cut them off below grade if you prefer to see the concrete instead of the forms.

For a 6x6 post what's the recommeded hole diameter? What size sonotube is recommended?

Using that Pythagorean guy's theorum, the corner to corner distance on a post with 5 1/2" nominal sides would be 7 3/4" (7.778 actually). To achieve at least 2" of concrete from the post corner to the side of the Sonotube would require a 12" diameter tube. Whether that is suitable or not is up for discussion.

How tall a fence are you building? 6 x 6 sunk 3.5' into the ground is awfully hefty for an ordinary fence. I replaced rotted posts in my fence in Louisiana with 4 x 6 posts sunk 2' into the ground w/ 5' above and no concrete. The 6" dimension was in line with the fence to provide maximum stability lateral to the fence. Depending on how your fence is designed, it can be very solid by itself in the fence line direction. If your soil packs solid you can tamp the dirt back around the post and not use concrete. All the concrete really does is gives a larger cross section against the soil to resist pushing over. IMO, the only time you really need concrete is with small metal posts, or maybe with 4 x 4 in sandy soil. Unless you have football player sized kids slamming against the fence, or 100 mph winds blowing on it, you will get plenty of lateral stability with 6" wide surface. I don't think the concrete buys much additional rot resistance over the PT. You will still get some water between the concrete and wood even with the top sloped, and the ground level is where most rot occurs.

Just my thoughts. I'm even thinking building my next fence with 2 x 6's for posts! Think about all the farm fences holding back cattle with bare posts in the ground. Cattle are not gentle on fence posts.

The fence will be 7 feet tall - 6 feet of boards + 1 foot of lattice. The guy drilling the holes is using a 10" auger which should yield a 12" hole. So I'll go with a 12" sonotube. Man this is going to be a lot of concrete!

Aaron I appluad you for building a nice hefty fence... I dont build fences because I am sure no one will pay for the fence that I would want to build. 12-14" Dia piers 4' deep with 6x6 posts @ 6' O.C. with 4x4 posts inbetween cut into and lag bolted with stainless steel bolts. Then the for the pickets using 2x6 cedar... Fence would never move but it would cost a small fortune.

So it looks like a slight change of plan - I'll be going with the 10" sonotube instead of the 12".

So my calculations for concrete needs are:

10" x 4' = 2.18 cubic feet to fill the hole, less:
6"x6"x4' = 0.85 cubic feet of space the post will take up when set in the hole, equals:

1.34 cubic feet per hole. A 30 kg (66 lb) bag of Quikrete Fence & Post should fill 0.495 cubic feet per bag which works out to 2.7 bags per hole. A 50 lb bag of Quikrete Fence & post will do .375 cubic feet so a 66 lb bag should do .495 cubic feet by my caluclations.

I'm about to order my materials, so hopefully someone can take a quick peek at this and let em know if I'm off base.

The Fast Set product specs that I'm looking at only shows it in 50lb (22.7kg) bags with a yield of .375 cu ft (11 L). That would put the quantity at 3.57 bags. Maybe they package it differently in Canada and that's the reason you have 66lb bags. None the less, your math is correct with the 66lb bag @ .495 cu ft yield resulting in 2.7 bags per post.

The Fast Set product specs that I'm looking at only shows it in 50lb (22.7kg) bags with a yield of .375 cu ft (11 L). That would put the quantity at 3.57 bags. Maybe they package it differently in Canada and that's the reason you have 66lb bags. None the less, your math is correct with the 66lb bag @ .495 cu ft yield resulting in 2.7 bags per post.

It is packaged differently up here, in 30 kg bags which works out to 66 lbs. All I did was figure out that if 50 lbs = .375, then 66 lbs SHOULD equal .495. Here's hoping.

On a side note, the "big box" store I'm ordering from indicated that each bag does 1 cubic foot. Suffice to say I ignored that info. Sigh...